In an automatic transmission, a torque converter capable of continuously transmitting torque of a power source from a stall state to a direct-coupled state is provided on a power transmission path between an internal combustion engine and a torque converter. There is a torque converter having a lock-up clutch that directly couples a pump impeller and a turbine runner to remove a rotation number difference between the internal combustion engine and the turbine runner so as to improve a fuel efficiency during driving when a rotation number difference between the pump impeller and the turbine runner is small. Further, among such torque converters, there is a torque converter having a mechanism (hereinafter, referred to as impeller clutch) that can separate the pump impeller from the internal combustion engine so as to reduce fluid resistance between the turbine runner and the pump impeller for the purpose of reducing fuel consumption during an idling.
Regarding control of such a vehicle having the impeller clutch in the torque converter, for example, JP-A-2009-115308 discloses a clutch operating method including, in order to achieve a turbo charge spool-up at the time of start moving, a step of increasing a first liquid pressure in an inside chamber from a first level to a second level, such that the first liquid pressure forces to move a clutch, which is provided between an engine of the vehicle and an impeller for a torque converter of the vehicle, to a connection position, and a step of decreasing a second liquid pressure in an outside chamber from a third level to a fourth level as a function of an engine speed, such that the second liquid pressure corresponds to the first liquid pressure and the second liquid pressure in the outside chamber is decreased as a function of a throttle position of the engine.
When the torque converter having the impeller clutch is applied to a vehicle with no turbo lag, it is required to engage the impeller clutch in a short time at the time of start moving, so as to improve a response. However, when the clutch operating method disclosed in JP-A-2009-115308 is applied to the vehicle with no turbo lag, the impeller clutch is controlled to output maximum torque with respect to a throttle opening degree. Accordingly, when the throttle opening degree is large and torque generated in the internal combustion engine is very high, a large torque fluctuation is transferred to the power transmission path of a rear side (transmission side) from the torque converter, so that shock is generated.
A need thus exists for a vehicle control apparatus that avoids shock being generated even when an impeller clutch is engaged in a short time in a vehicle with no turbo lag.